1. Field of the Invention
This invention generally relates to a method and system for non-invasively measuring pulmonary function, and more particularly to a method and system that provides quantitative descriptions of lung volumes which are exchanging air more slowly than the bulk of the lung volume.
2. Related Art
The importance of characterizing the complexity of pulmonary function in diagnosing asthma and in assessing potential treatments for asthma has been reviewed recently in the literature. See, e.g., S. Wagers, “Polarized Helium: Changing Our View of Asthma,” J. Allergy Clin. Immun. 1201 (June 2003). Characterizing the complexity of pulmonary function is also useful in diagnosing and in assessing potential treatments for Chronic Obstructive Pulmonary Disease (COPD), as well as many other pulmonary conditions which are familiar to medical experts.
The most common clinical test for asthma is the Forced Expiratory Volume in One Second (FEV1) test. The FEV1 test is often used in conjunction with the Forced Vital Capacity (FVC) test. For example, NIH guidelines specify that a FEV1 value <80% of the predicted value, or a FEV1/FVC value <65% of the predicted value, is an appropriate method for diagnosing asthma. See NIH publication no. 97-4053, Practical Guide for the Diagnosis and Management of Asthma, (1997). The precision of such measurements is about 10%. While a real correlation between the diagnostic thresholds and clinical expression of symptoms exists, most children with asthma have FEV1 values above the NIH threshold value. See J. Spahn, et al., “Is Forced Expiratory Volume in One Second the Best Measure of Severity in Childhood Asthma?,” Am. J. Respir. Crit. Care Med. 169:784 (2004). The FEV1 and FVC tests are also used to diagnose COPD and other pulmonary disorders.
While clinical tests exist for identifying patients suffering from asthma, COPD, and related pathologies, medical practitioners and investigators have identified the need for better tests to more correctly identify the nature and level of small airway constriction or blockage both in diagnosing these pathologies and in assessing the benefits of the treatments of these pathologies. Medical practitioners and investigators have also identified the need for better measures to quantify pulmonary pathologies.